Images are created, edited, viewed, and used in other ways by countless number of devices and applications. Increasingly, annotating images through ink applications (i.e. taking notes, editing, modifying existing text on images, and so on) and devices such as tablet PCs, handheld computers, and so on, is becoming popular and common. However, there are still a very large number of applications in use that are annotation-unaware (i.e. they cannot decode annotation information on an annotated image).
To store annotations in an image file two methods are commonly used. The first is to flatten the annotations directly onto the image. Flattening is a term used for integrating annotations or other types of additional information such as additional layers of image into a single layer where all additional information becomes an integral of the original image possibly obscuring portions of it. The second is to embed the ink data in a secondary location within the image file (such as the metadata headers, a side channel, etc.) and have only specific annotation-aware applications be able to display the annotations.
There are significant drawbacks to both approaches. Flattened images can be shared very easily—the annotations will always be displayed along with the picture since they are part of the picture. However, the information for the obscured portion of the image may be forever lost and cannot be easily (or not at all) reconstructed. On the other hand, embedded ink allows preservation of the original image data in that it is non-destructive to the image content and can easily be edited/removed at a later date. However, since embedded ink is not part of the image content, it can only be displayed by a limited set of annotation-aware applications.